1. Field of the Invention
The present invention relates to a wavelength division multiplexing device and, more particularly, to an add/drop multiplexer for adding or dropping channels to/from an optical signal.
2. Description of the Related Art
Wavelength division multiplexing is a technique used to simultaneously transmit a plurality of channels with different wavelengths on a single waveguide, whereas wavelength division demultiplexing is a technique used to separate a group of multiplexed channels into an independent channel. The demand for using a wavelength division multiplexing system in optical communication networks is growing to accommodate the increase in communication capacity. For this type of optical communication network, add/drop multiplexers are typically used to achieve the function of adding new channels or dropping unused channels. For concurrent transmission of optical signals in forward and reverse directions, a bidirectional add/drop multiplexer has been used as it provides a combined function of two unidirectional add/drop multiplexers. However, such a bidirectional add/drop multiplexer has a high probability of generating undesirable relative intensity noise due to its configuration in which a signal wavelength division multiplexer/demultiplexer is typically used.
FIG. 1 is a diagram illustrating the generation of relative intensity noise at one of the connectors in a conventional bidirectional add/drop multiplexer. As shown in FIG. 1, the bidirectional add/drop multiplexer includes first and second connectors 12 and 19 for providing a connection between an external waveguide 11 and an internal waveguide 13, first and second circulators 16 and 18 for branching an input optical signal 14 to desired terminals, respectively, and a 16×16 arrayed waveguide grating 17. The input optical signal 14 is the optical signal of a single channel.
The first circulator 16 serves to input the optical signal 14 of a single channel, which travels in a forward direction, to the 11-th terminal of the arrayed waveguide grating 17. The 11-th terminal of the arrayed waveguide grating 17 is a forward demultiplexing terminal and serves the optical signal 14 to the 10′-th terminal. This 10′-th terminal is connected to the 16′-th terminal. The optical signal 14 outputted from the 16′-th terminal is applied to the 5-th terminal, which is a forward multiplexing terminal. The optical signal 14 from the 5-th terminal is applied to the second circulator 18, which in turn transmits the optical signal 14 to the second connector 19 connected to the external waveguide 11.
In the above arrangement, the relative intensity noise may be generated easily due to the following three main causes.
The first cause is the Rayleigh back-scattering phenomenon of an optical signal with a plurality of channels that occurs as the optical signal travels along a single waveguide. Typically, the Rayleigh back-scattering phenomenon occurs due to structural defects in the waveguide.
The second cause is a reflection phenomenon that occurs in a connector. Typically, this reflection phenomenon occurs due to the non-uniformity of the boundary between the internal and external waveguides of a bidirectional add/drop multiplexer.
The third cause is a cross-talk phenomenon of optical signals that occurs in an arrayed waveguide grating. That is, although the channels of an optical signal should be inputted to designated terminals during demultiplexing and multiplexing operations, respectively, they may be unintentionally inputted to adjacent terminals that are located around the designated terminals.
Furthermore, where the relative intensity noise exists on the same path as that of the channel with the same wavelength, the interference may occur in that channel.
With continued reference to FIG. 1, the relative intensity noise 15 generated at the first connector 12 travels the same path as that of the optical signal 14 inputted to the first connector 12, and enters the 5′-th terminal via the 11-th terminal. The relative intensity noise 15 from the 5′-th terminal is inputted to the first circulator 16, which in turn outputs the relative intensity noise 15 back to the first connector 12. Thus, the relative intensity noise 15 is circulated repeatedly.
FIG. 2 is a diagram illustrating the generation of relative intensity noise at the second connector in the conventional bidirectional add/drop multiplexer. As shown in FIG. 2, the relative intensity noise 25 generated at the second connector 29 is inputted to the 11′-th terminal, which is a reverse demultiplexing terminal, after passing through the second circulator 28. The relative intensity noise 25 from the 11′-th terminal is inputted to the 5-th terminal, which is a reverse multiplexing terminal. Thereafter, the relative intensity noise 25 from the 5-th terminal is inputted to the second circulator 28, which in turn outputs the relative intensity noise 25 to the second connector 29. Thus, the relative intensity noise 25 is circulated repeatedly.
The relative intensity noise described with reference to both FIGS. 1 and 2 may be generated at the first and second connectors and may thus travel simultaneously. In addition, the forward and reverse optical signals may collide with each other at a particular terminal. In such a case, the wavelength corresponding to the terminal associated with the signal collision cannot be transmitted. As a result, the number of channels available in the add/drop multiplexer is reduced. In order to avoid this type of channel reduction, it is necessary to install an additional circulator at the terminal associated with the signal collision or to replace the circulator associated with the signal collision a 4-terminal circulator.
As mentioned above, the conventional bidirectional add/drop multiplexer has drawbacks in that the relative intensity noise may be generated and the forward and reverse optical signals may collide with each other at a particular terminal.